Carboxylic acidsEdexcel International A Level Chemistry: Revision notes
Section 1
Naming and structure
Carboxylic acids contain the carboxyl group, –COOH, with the carbon doubly bonded to one oxygen and singly bonded to an –OH. The carboxyl carbon is always carbon 1, so the name ends -oic acid with no number: methanoic acid HCOOH, ethanoic acid CH₃COOH, propanoic acid CH₃CH₂COOH, 2-methylpropanoic acid (CH₃)₂CHCOOH. Draw structural, displayed and skeletal formulae; in skeletal form –COOH shows as a branch with =O and –OH.
In a skeletal formula, count the carbon of the –COOH group when naming: CH₃CH₂COOH has three carbons and is propanoic acid.
Section 2
Hydrogen bonding, boiling temperature and solubility
The –COOH group has an O–H that can form hydrogen bonds and a C=O oxygen with lone pairs. Carboxylic acid molecules form extensive hydrogen bonds with each other (often pairs of molecules, dimers), so their boiling temperatures are higher than those of alcohols and aldehydes of similar Mr. Ethanoic acid (Mr 60) boils at 118 °C, propan-1-ol (Mr 60) at 97 °C.
Carboxylic acids form hydrogen bonds with water, so short-chain acids (methanoic to butanoic) are miscible with water. Solubility falls as the non-polar hydrocarbon chain gets longer.
Section 3
Making carboxylic acids
Oxidation of primary alcohols: heat under reflux with an excess of acidified potassium dichromate(VI). The reflux condenser returns the volatile aldehyde to the flask so that it is oxidised further. Orange turns to green.
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O
Oxidation of aldehydes: CH₃CHO + [O] → CH₃COOH.
Hydrolysis of nitriles: heat under reflux with dilute acid, for example
CH₃CN + 2H₂O + HCl → CH₃COOH + NH₄Cl
This route lengthens the carbon chain by one carbon, because a halogenoalkane can first be converted into a nitrile with KCN.
Distilling the mixture during the oxidation. That removes the aldehyde and gives the aldehyde as the product, not the acid.
Section 4
Reactions as acids
Carboxylic acids are weak acids: they partly ionise in water, RCOOH ⇌ RCOO⁻ + H⁺. They react with bases to form salts:
- with carbonates: 2CH₃COOH + Na₂CO₃ → 2CH₃COONa + H₂O + CO₂ (effervescence, a test for –COOH)
- with alkalis: CH₃COOH + NaOH → CH₃COONa + H₂O
- with reactive metals: hydrogen is given off.
The salts are named -oate: sodium ethanoate.
Section 5
Reaction with PCl₅ and alcohols
Phosphorus(V) chloride converts –COOH into an acyl chloride, with steamy fumes of HCl:
CH₃COOH + PCl₅ → CH₃COCl + POCl₃ + HCl
Heating a carboxylic acid with an alcohol and a few drops of concentrated sulfuric acid as catalyst gives an ester and water, in a reversible reaction:
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
The ester name comes from the alcohol (alkyl) then the acid (-oate): ethyl ethanoate.
Naming the ester the wrong way round. The alcohol part comes first: propanoic acid with ethanol gives ethyl propanoate.
Section 6
Reduction with LiAlH₄
Lithium tetrahydridoaluminate(III) in dry ether reduces a carboxylic acid to a primary alcohol, using [H] in the equation:
CH₃COOH + 4[H] → CH₃CH₂OH + H₂O
The ether must be dry because LiAlH₄ reacts violently with water. Weaker reducing agents do not reduce carboxylic acids.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Carboxylic acids
- Ethanoic acid (Mr 60) boils at 118 °C, whereas propan-1-ol (Mr 60) boils at 97 °C and propanal (Mr 58) boils at 49 °C. Ethanoic acid is fully miscible with water.Explain why the solubility of carboxylic acids in water decreases as the length of the carbon chain increases.2 marks
- A chemist prepares ethanoic acid by two different routes. In the first, ethanol is heated under reflux with an excess of acidified potassium dichromate(VI). In the second, ethanenitrile is heated under reflux with dilute hydrochloric acid.Write a balanced equation for the hydrolysis of ethanenitrile by dilute hydrochloric acid.2 marks
- Propanoic acid is reacted separately with (i) aqueous sodium carbonate, (ii) phosphorus(V) chloride, (iii) ethanol with a few drops of concentrated sulfuric acid, heated under reflux, and (iv) lithium tetrahydridoaluminate(III) in dry ether.Describe what is seen in reaction (i) and name the organic product of reaction (ii). Write a balanced equation for reaction (i).3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).